CPEB is a sequence-specific RNA binding protein that regulates polyadenylation-induced translation during vertebrate development. While infertile, a global CPEB knockout mouse is viable and normal size. However, mouse embryo fibroblasts (MEFs) derived from these knockout (KO) animals are immortal whereas MEFs derived from wild type (WT) animals, as expected, become senescent after 7-9 passages in culture. Infection of early passage KO MEFs with a virus expressing CPEB rescues the senescence phenotype;infection of late passage immortalized KO MEFs has no effect. Moreover, infection of early passage WT MEFs with a CPEB-expressing virus induces premature senescence while infection of immortalized WT MEFs has no effect. These results demonstrate that CPEB is necessary for cell senescence. Additional experiments indicate that i.) CPEB regulation of cell senescence requires the tumor suppressors p53 and p19ARF, ii.) CPEB-mediated translation controls senescence, and iii.) the CPEB KO MEFs may be partially transformed. The specific aims of this proposal include the determination of the underlying molecular and cellular mechanisms by which CPEB regulates senescence in mouse cells. Additional evidence is presented indicating that CPEB also controls senescence in human cells. Many of the techniques we will employ are those we have used for several years to study CPEB-regulated translation and include an analysis of RNA-protein interactions, protein-protein interactions, examination of poly(A) tail length, and immuno-cytochemistry. These proposed experiments deal directly with cancer etiology and the ageing process, which is central to the mission of NIH. This proposal focuses on new molecules that regulate cell senescence. Because this process is strongly linked to cancer and ageing, it clearly has important implications for human health.